In composite forming fabrics that include two essentially separate woven structures, the paper side layer is typically a single layer woven structure which provides, amongst other things, a minimum of fabric mark to, and adequate drainage of liquid from, the incipient paper web. The paper side layer should also provide maximum support for the fibers and other paper forming solids in the paper slurry. The machine side layer is also typically a single layer woven structure, which should be tough and durable, provide a measure of dimensional stability to the composite forming fabric so as to minimize fabric stretching and narrowing, and sufficiently stiff to minimize curling at the fabric edges. It is also known to use double layer woven structures for either or both of the paper and machine side layers.
The two layers of a composite forming fabric are interconnected by means of either additional binder yarns, or intrinsic binder yarns. The chosen yarns may be either warp or weft yarns. The paths of the yarns are arranged so that the selected yarns pass through both layers, thereby interconnecting them into a single composite fabric. Examples of prior art composite forming fabrics woven using intrinsic binder warp or weft yarns are described by Osterberg, U.S. Pat. No. 4,501,303; Bugge, U.S. Pat. No. 4,729,412; Chiu, U.S. Pat. No. 4,967,805, U.S. Pat. No. 5,219,004 and U.S. Pat. No. 5,379,808; Givin, U.S. Pat. No. 5,052,448; Wilson, U.S. Pat. No. 4,987,929 and U.S. Pat. No. 5,518,042; Ward et al, U.S. Pat. No. 5,709,250; Vohringer, U.S. Pat. No. 5,152,326; Johansson, U.S. Pat. No. 4,605,585; Hawes, U.S. Pat. No. 5,454,405; Wright, U.S. Pat. No. 5,564,475; and Seabrook et al, EP 0 794 283. A major difference between intrinsic binder yarns and additional binder yarns is that additional binder yarns do not contribute significantly to the fundamental weave structure of the paper side surface of the paper side layer, and serve mainly to bind the two layers together. Additional binder yarns have been generally preferred over intrinsic binder yarns for commercial manufacture of composite forming fabrics because they were thought to be less likely to cause discontinuities, such as dimples, in the surface of paper side layer. Examples of prior art fabrics woven using additional binder yarns are described by Johansson et al., CA 1,115,177; Borel, U.S. Pat. No. 4,515,853; Vohringer, DE 3,742,101 and U.S. Pat. No. 4,945,952; Fitzka et al, U.S. Pat. No. 5,092,372; Taipale, U.S. Pat. No. 4,974,642; Huhtiniemi, U.S. Pat. No. 5,158,117; and Barreto, U.S. Pat. No. 5,482,567.
In composite forming fabrics where intrinsic warp binder yarns from the machine side layer have been used to interconnect the paper and machine side layers, the prior art has generally advocated modifying the path of the selected machine side layer warps so as to bring these yarns up to the paper side layer to interlace with it at selected weft knuckles. A known disadvantage associated with this practice is that the area immediately adjacent these tie locations tends to become pulled down into the fabric structure, well below the plane of the adjacent knuckles, causing a deviation in the paper side surface of the paper side layer, commonly referred to as a "dimple". These dimples frequently create a pronounced unevenness in the paper side surface of the fabric, which can result in an unacceptable mark in any paper formed on the fabric.
In comparison, intrinsic weft binder yarns have been found to cause less paper side surface dimpling, and hence have been a preferred method of interconnecting the layers of composite forming fabrics. However, there are a number of problems associated with their use.
First, intrinsic weft binder yarns have been found to cause variations in the cross-machine direction mesh uniformity of the paper side surface of the paper side layer in certain weave patterns. This can create an unacceptable level of marking in some grades of paper.
Second, fabrics woven using intrinsic weft binder yarns are known to be susceptible to lateral contraction, or narrowing, when in use. Lateral contraction may be defined as the degree to which a fabric narrows when machine direction (or longitudinal) tension is applied. If the fabric narrows excessively under this tension, particularly at driven rolls in the forming section, the resulting width changes will cause the fabric to buckle or form ridges. Generally, single layer fabrics, and composite fabrics having additional or intrinsic weft binder yarns, exhibit much higher degrees of lateral contraction than either double layer, or extra-support double layer, fabrics of comparable mesh.
Third, composite forming fabrics containing intrinsic weft binder yarns are less efficient to weave than comparable intrinsic warp binder designs, because a greater number of weft yarns is required to provide a reliable interconnection between the paper side layer and the machine side layer. Comparable fabrics whose designs utilize intrinsic warp binder yarns require fewer weft yarns per unit length, since none of the weft yarns is utilized to interconnect the paper and machine side layers. For example, a fabric containing intrinsic warp binder yarns whose paper side layer is woven so as to provide 31.5 weft yarns/cm, and 15.75 weft yarns/cm on its machine side layer (resulting in a 2:1 ratio of the paper side layer to machine side layer weft yarn count), has a total weft yarn count of 47.25 yarns/cm. A comparable intrinsic weft binder yarn fabric, woven at 31.5 weft yarns/cm in its paper side layer and which employs additional weft yarns to interconnect the layers, has a total weft yarn count of between 55 to 63 weft yarns/cm, depending on the paper side layer to machine side layer weft yarn ratio, because additional weft yarns must be provided so as to tie the two layers together. A comparable fabric utilizing intrinsic warp binder yarns requires up to 25% fewer weft yarns to weave each unit length.
Fourth, a fabric utilizing intrinsic warp binder yarns will generally have a lower caliper (and thus be thinner and provide a lower void volume) than a comparable fabric of similar specification utilizing intrinsic weft binder yarns. Because there are fewer weft yarns per unit length, those remaining do not contribute as much to the thickness of the fabric.
A benefit provided by composite fabrics utilizing intrinsic warp binder yarns is their increased resistance to delamination, when compared to a composite fabric utilizing either additional or intrinsic weft binder yarns. Delamination, which is the catastrophic separation of the machine and paper side layers, is generally caused by one of two mechanisms. The first is abrasion of the binder yarn where it is exposed on the machine side of the fabric as it passes in sliding contact over the various stationary elements in the forming section. In composite fabrics utilizing intrinsic warp binder yarns, it is possible to recess the warp binder yarns relative to the wear plane of the fabric to a greater degree (e.g. by as much as 0.05-0.076 mm) further away from the wear plane than is possible in a comparable fabric utilizing intrinsic weft binder yarns. This means that more machine side layer warp and weft yarn material must be abraded away from the running side of a fabric utilizing intrinsic warp binder yarns before the tie strands are broken, and the two layers delaminate, than in a comparable fabric utilizing intrinsic weft binder yarns.
The second delamination mechanism, which is encountered more rarely than the first, is that of internal abrasion of the binder yarns between the machine and paper side layers as they flex or shift relative to one another. The presence of abrasive fillers in the stock, such as clay, titanium dioxide and calcium carbonate greatly exacerbates the rate of this type of abrasion. Composite forming fabrics whose paper and machine layers are well interlaced so as to prevent or reduce relative movement of these layers (such as in the fabrics of the present invention utilizing intrinsic warp binder yarns) will experience less internal abrasion than comparable fabrics utilizing intrinsic weft binder yarns. They are therefore less susceptible to delamination by internal abrasion.
Accordingly, the present invention seeks to provide a composite forming fabric whose construction is intended at least to ameliorate the aforementioned problems of the prior art.
The present invention further seeks to provide a composite forming fabric having reduced susceptibility to cross-machine direction variations in the paper side layer mesh uniformity than comparable fabrics of the prior art.
Additionally, this invention seeks to provide a composite forming fabric that is resistant to lateral contraction.
This invention also seeks to provide a composite forming fabric that is more efficient to weave than comparable fabrics utilizing intrinsic weft binder yarns to interconnect the paper and machine side layer woven structures.
Furthermore, this invention seeks to provide a composite forming fabric that is less susceptible to dimpling of the paper side surface.
In a preferred embodiment, this invention seeks to provide a composite forming fabric having a lower void volume than a comparable forming fabric utilizing intrinsic weft binder yarns.
This invention additionally seeks to provide a composite forming fabric that is resistant to delamination.